1. Field of the Invention
The present invention relates to a process for producing a low-polarization mode dispersion optical fiber.
2. Description of the Related Art
Optical signals transmitted through single-mode optical fibers comprise two orthogonal polarization modes (typically denoted TE and TM) that, in case of a fiber with a perfectly cylindrical core of uniform diameter, propagate at a common velocity. However, in real optical fibers the core cylindrical symmetry may be disrupted due to shape defects or non-uniform stresses. As a result, a phase difference can accumulate between the two modes, and the fiber is said to show “birefringence”. In particular, the birefringence introduced by shape and stress asymmetry is known as “intrinsic linear birefringence”.
The structural and geometrical irregularities of the optical fiber that give rise to birefringence typically originate from the preform itself and are modified during the process of drawing the fiber. This process is usually carried out by means of an apparatus known as a “drawing tower”, starting from a glass preform. In practice, after the preform has been placed in a vertical position and heated to a temperature above the softening point within a suitable furnace, the molten material is drawn downwards at a controlled velocity in such a way as to produce a threadlike element that forms the optical fiber itself. In this process, asymmetrical stresses are typically applied to the fiber.
In a birefringent fiber, the two components of the fundamental mode, initially in phase with each other, come to be in phase again only after a certain length, commonly known as the “beat length”. In other words, the beat length is the period of repetition of a certain state of polarization (on the assumption that the fiber maintains a constant birefringence over this length).
In the so-called “polarization-preserving fibers”, asymmetry is deliberately introduced into the fiber to generate birefringence. However, in ordinary (i.e. non-polarization-preserving) fibers, birefringence is detrimental. In fact, when pulsed signals are transmitted into an optical fiber, the birefringence is a potential cause of pulse spreading, since the two polarization components excited by the pulses travel at different group velocities (i.e. become dispersed). This phenomenon, known as polarization mode dispersion (PMD), has been widely studied in recent years because of its importance in periodically amplified light guide systems.
Typically, the phenomenon of PMD leads to a limitation of the width of the signal transmission band and, consequently, a degradation of the performance of the optical fibers along which the aforesaid signals are transmitted. This phenomenon is therefore undesirable in systems of signal transmission along optical fibers, especially in those operating over long distances, in which it is necessary to minimize any form of attenuation or dispersion of the signals to guarantee high performances in transmission and reception.
U.K. patent application GB2101762A considers the effects on PMD of post-draw fiber twisting and observes that, although this twisting reduces the PMD resulting from intrinsic linear birefringence, it introduces torsional stresses that generate a substantial circular birefringence due to the photo-elastic effect. Twisting a drawn fiber thus reduces the bandwidth limitation due to one effect whilst replacing it with another. The same patent application thus propose to spin the preform during drawing, so that twisting can be effected whilst keeping the fiber material substantially unstressed. Spinning is performed at a relatively high rate, so that its spatial repetition frequency, or spin pitch, is small compared to the beat length due to intrinsic birefringence; as a result, an optical fiber is formed wherein the contribution of birefringence due to form and stress asymmetry is greatly reduced. Such a fiber is termed “spun” fiber to distinguish it from a twisted fiber. Conveniently the preform is spun at a substantially constant rate, but it could even reverse in direction, oscillating from a right-handed to a left-handed twist.
Due to spinning, the fiber undergoes a rotation of its polarisation axes. As a result, when the optical pulses are transmitted into the optical fiber, they propagate alternately on the slow and fast birefringence axes, thus compensating the relative delay and reducing the pulse spreading. This is equivalent to have a local effective refractive index for the pulses equal to the mean refractive index on the two axes, the average being taken over the pulse length along the fiber.
Theoretical studies have been carried out to analyse the effects of spinning on birefringence. For example, the article of A. J. Barlow et al., “Anisotropy in spun single-mode fibers”, Electronics Letters, 4 Mar. 1982, Vol. 18, No. 5, shows that the dominant process for the reduction of PMD in a spun fiber is the averaging of the local fiber anisotropy by the rapid procession of the axes of asymmetry along the fiber. A further theoretical contribution is provided by the article of M. J. Li, “Effects of lateral load and external twist on PMD of spun and unspun fibers”, ECOC '99, 26-30 Sep. 1999, Nice, France, which proposes a novel model (based on the coupled mode theory) to analyse the effects of lateral loads and external twist on PMD of spun and unspun fibers.
Apart from theoretical considerations, several techniques have been developed to reduce the PMD of an optical fiber, which take advantage of the effects of spinning during drawing.
Similarly to U.K. patent application GB2101762A, U.S. Pat. No. 5,581,647 describes a method of fabricating a dispersion compensation optical fiber, wherein the fiber is drawn while the preform is rotated about its axis. Particular process conditions are herein described: the fiber shall be drawn at a speed between 50 and 1,000 m/min and at a tension between 2.4 and 13 kg/mm2, and the preform rotated at a speed between 10 and 1.000 rpm.
International patent application WO 97/26221 observes that the reduction in PMD produced by spinning is proportional to the spin rate, that very high spin rates are generally required to deal with the asymmetries of typical fibers, e.g., spin rates greater than 5,000 rpm, and that spinning a preform at such rates is not a practical solution for commercial fiber production.
U.S. Pat. No. 4,504,300, relating to a technique for making an optical fiber having chiralic structure, addresses drawbacks related to preform rotation and proposes a new spinning technique, consisting in rotating the fiber instead of the preform. In particular, a device is disclosed comprising means disposed just below the preform for twisting the fiber during fiber drawing. The twisting means comprise a rotating hoop supporting three pulleys. The twisted fiber is coated by coating means, followed by cooling by fast-cooling means that facilitate freezing-in of the twist.
As observed in WO 97/26221, a drawback associated to this technique is connected to the high risk of damaging the fiber surface due to the fact that the latter gets in touch with the above-mentioned pulleys before being properly protected by a suitable coating film.
In order to overcome this drawback, U.S. Pat. No. 5,298,047 and U.S. Pat. No. 5,418,881 propose to arrange the device adapted to apply the torque to the fiber downstream of the coating station. In particular, in the techniques herein described, the torque is applied by alternately canting in clockwise and counterclockwise direction a fiber guiding roll having a rotation axis which extends perpendicularly to the drawing axis of the fiber.
US patent application N. 2001/0020374 observes that, although substantially achieving the object of reducing the PMD of the fiber, the use of a canting roll shows a series of problems (for example the need of limiting the canting frequency to avoid a relative sliding between fiber and roll) and is only suitable for the application of an alternate (i.e. bidirectional) spinning to the fiber. However, although a new device is proposed that overcomes the drawbacks of the canting-roll technique and allows both unidirectional and alternate spinning, alternate spinning is still considered as preferable since it prevents the presence of residual torsions on the fibers wound onto the collecting spool, thus making easier both the unwinding and wiring operations of the same.
The possible presence of unwanted elastic twist is also noticed in U.S. Pat. No. 5,298,047, in relation to the application of an intermittent torque to the fiber. Apart from stating that it is generally desirable to remove the elastic twist, for instance by appropriate respooling, the Authors of that patent consider as preferable to alternately impose a clockwise and a counteclockwise torque to the fiber to substantially prevent introduction of the elastic twist.
The Applicant observes that alternate spinning has several drawbacks not previously highlighted. Alternate spinning may for example cause a relatively low mechanical efficiency of the spinning device, due to the continuous accelerations and decelerations. Moreover, with respect to a unidirectional spin, an alternate spin requires a relatively high peak profile amplitude to compensate those positions of the profile where the rotation slows down to change direction and, therefore, to guarantee a sufficient average spin rate. Besides, the sites where the spin rate is zero are detrimental for the PMD because there is an increase of the effective birefringence seen by the pulse, and so a higher contribution for PMD.
The Applicant has therefore considered again the use of unidirectional spin, and tried to solve the problems associated thereto. The Applicant has verified that, to achieve an efficient PMD reduction, a technique making use of unidirectional spin must take into account the effects of spin and of the circular birefringence induced by elastic twist as well.